The invention relates to a method for the coating of a metallic article with a polymer composition which comprises multiply unsaturated compounds, and also to the coated metallic articles themselves and to their use. The coated metallic articles may be, for example, metal pipes clad with the polymer composition.
Metallic articles intended for applications in which they are exposed to corrosion-promoting environmental influences may be coated with a protective layer of polymer materials, this layer being applied to the metallic article in the form of a melt, by means of extrusion coating, for instance. This not only blocks access to the metal surface for the corrosion-promoting agent, but also provides the surface with some mechanical protection with respect, for example, to stresses such as friction, impact or jolt. This protection is desirable even when the surface has also undergone corrosion control treatment prior to coating. A vital prerequisite for such protection is that the applied coating adheres strongly, so that in the event of mechanical damage, the corrosion-promoting agent—for example, water or aqueous salt solution—does not migrate beneath the coating.
If the metallic article is composed, for example, of aluminium, an aluminium alloy, zinc or galvanized steel, it has conventionally been pretreated by chromating. This generates a passivating coat which has the effect of improving not only the corrosion resistance but also the adhesion of a subsequently applied polymer composition. In the wake of new statutory guidelines such as the EU End of Life Vehicles Directive and the RoHS Directive, chromation with carcinogenic chromium(VI)-containing compounds is in many sectors no longer permissible, however. This has given rise to a great interest in chromium(VI)-free alternatives. Examples of such alternatives are chromium(III)-passivations, but these are weaker by comparison, or the use of passivating baths which comprise titanium(IV)-salts and/or zirconium(VI)-salts, such as complex fluorides, for example, with which a passivating coat of titanium oxide or zirconium oxide, respectively, is formed. In passivating baths of these kinds it is also possible for the use of other metal salts, such as cobalt salts or cerium salts, for example, to play a part. As requirements become more acute, the passivating coat obtained may serve as an adhesion base for coating with a thermoplastic or thermoset polymer system, thereby further improving the corrosion control.
Another possibility for corrosion control is phosphating. Phosphating is a widespread technique within surface technology, in which a layer of strongly adhering metal phosphates is formed by chemical reaction of metallic surfaces with aqueous phosphate solutions and/or aqueous phosphoric acid. Phosphating is mostly employed with steel, but can also be used for galvanized or cadmium-plated steels and aluminium. Phosphate layers alone offer a useful temporary protection from corrosion, which is often sufficient for storage prior to a subsequent processing step. In the art, phosphate layers are used very frequently as a base for further, subsequent coatings.
In all of these cases, the strong adhesion of the subsequent coatings to the passivating coat or phosphate layer is of considerable importance. This strong adhesion is vital for effective prevention of oxygen and electrolyte penetration that those locations where mechanical damage occurs to the coating, as a result of stonechipping, for example, in order that filiform corrosion does not occur. Filiform corrosion is a type of corrosion which occurs for example on steel or aluminium beneath organic coats. It is characterized by threadlike subfilm migration. It is normally caused by a defect site or scratch in the coat. Strong adhesion between coating and metal must also be ensured so that further processing steps such as crimping or bending are not accompanied by any delamination of the coating, which would likewise result in subsequent filiform corrosion.
EP 1 130 066 A1 discloses an extrusion-coated metal profile for which an organosilane composition serves as an adhesion base for a coat which is applied by extrusion and consists of a polyamide moulding compound.
WO 2009/027429 discloses the use of a metallic line pipe whose surface is clad with a polyamide coat, for producing a pipeline which is laid trenchlessly or without a sand bed.
WO 2010/094528 discloses the use of a metallic line pipe which is clad with an extruded coat of a polyamide moulding compound for producing a pipeline which is laid in water.
U.S. Pat. No. 5,520,223 describes a zinc-plated metal pipe whose surface, following a pretreatment, is coated with a moulding compound based on thermoplastic elastomers, ionomers, polyamides and/or fluoropolymers.
In the described conventionally known systems, the coating material makes only a passive contribution to corrosion control, by hindering the ingressive water, including salt water, and atmospheric oxygen. Diffusion of atmospheric oxygen through the coating, however, continues to be possible.
In view of the above described problems, the object of the present invention was to provide a polymer composition for the coating of metallic articles that exhibits enhanced adhesion both to untreated and to treated metal surfaces and at the same time makes an active contribution to corrosion control.